1. Field of the Invention
The invention relates to generation of switching signals for the chroma keying of color video signals.
2. Description of the Prior Art
Chroma keying methods for background blending or trick mixing in color television studios are known per se. In a known method, the so-called self controlled chroma keying method, the control of a switch is carried out by means of a switching signal in such a manner that a foreground scene in front of, for instance, a blue background, is taken with high color saturation, and the occurrence of a color signal with a high blue content serves as criterion for the actuation of the switch. The foreground scene must in this case contain no "highly saturated" blue.
From British Pat. No. 1,189,552, a remotely controlled circuit arrangement for the chroma keying of color video signals is known. This circuit arrangement for chroma keying supplies a switching signal at the occurrence of the selected hue. The switching signal is produced by means of two automatic volume-control amplifiers and an adding circuit. The automatic volume-control amplifiers are continuously adjustable by means of a sine-shaped and cosine-shaped signal, so that any hue in the color signal of a color video source can be selected. When the amplitude of the selected hue exceeds a set threshold value, an electronic switch is actuated which switches the operation, for instance, from one television camera to another one. When, on the other hand, the amplitude of the selected color drops below the set threshold value, the original camera is reconnected by the switch.
This known method, however, presents the disadvantage that hues deviating from the selected hue neighboring color tones generate a switching signal when the amplitude of the selected hue is small, that is to say, when the selected hue exhibits only a small color saturation, and the threshold value must be set at a corresponding low value. Thus, e.g. in a selected hue "blue", the fully saturated mixed colors cyanic and purple with 50% of the total amplitude of blue are contained. In order to increase the hue selection of such a known circuit arrangement, it has already been proposed to feed to one input of an AND gate a signal component of the value (B-M) sin x - (R-M) cos x, and to another input of the AND gate a signal component of the value (B-M) cos x + (R-M) sin x, in which values x corresponds to the hues corresponding to 0.degree. - 360.degree. in color spectrum. The AND gate furnishes an output signal only when both signals are negative. At x = 0.degree., the resulting values are -(-R-M) and (B-M). This means in the present case that, in a coordinate system with a (R-M) axis and a (B-M) axis, colors in the yellow-red quadrant become negative and will therefore be keyed out. By this measure, to be sure, the color selection is increased, but adjacent colors in the quadrant of the selected hues will, when the threshold value of the selected hue is set at a low level, continue to influence the switching signal.
The present invention therefore provides a method for generating a switching signal for the chroma keying of color signals of the type initially mentioned, in which method the color selection is increased. Furthermore, the hue selection is independent of the set degree of color saturation.
According to the invention, to set a hue-selection degree which is independent of the hue selected, a second signal component with adjustable amplitude is subtracted from the first signal component. In this process, the second signal component for the selected hue equals zero and differs from zero with respect to the adjacent colors.
In one embodiment of the invention, the first color difference signal is multiplied by the cosine-shaped control signal, and the second color difference signal is multiplied by the sine-shaped control signal. These multiplied signals are subtracted from each other and constitute, after the corresponding value is formed, a second signal component. Furthermore, the second signal component with adjustable amplitude is subtracted from the first signal component. The signal thus produced is supplied to a threshold value switch.
In a further development of the invention, the second signal component is formed such that the subtracted multiplied signals that differ from the subtracted multiplied signals by opposite signs are mixed in a non-additive manner at the same blanking level.
The invention now allows the degree of hue selection to be chosen independently of the hue set. This is accomplished because the second signal component for the selected hue itself becomes always zero. An amplitude change of the second signal component therefore affects only adjacent hues.